Pick the protocol from the url if provided

[pmlopes]

Currently, the client negotiates the protocol version at connection level, or falls back to 2 if configured so.

Instead, we could pick it from the url if provided, as it is now a reserved property.

redis/redis-specifications#12 (review)

[Ladicek]

I wish I noticed this issue sooner, I would have used the protocol param instead of adding an extra method in #422. Oh well. Maybe we can do both? 🤷